1. Field of the Invention
The present invention relates to a music data compression method of compressing performance event information formed of note information, such as information of a tone pitch, sounding timing, sounding duration, and a channel number corresponding to a part, and more particularly to a music data compression method suitable e.g. for distribution of music data, such as an incoming call melody for a cellular phone.
2. Description of the Related Art
In recent years, the use of networks by electronic devices or apparatuses (terminal devices), such as cellular phones and personal computers, has been rapidly expanding, and it has become possible to receive services of various data contents from servers via such terminal devices. An example of the data contents includes music data to be sounded as an incoming call melody through a cellular phone, and music data of a music piece or karaoke music played through a personal computer.
However, as the reproduction time and/or the number of parts of such a music piece increases, the data size also increases, which causes an increase in communication time and costs necessary for downloading music data of an incoming call melody or the like. Further, a terminal device necessitates a large memory capacity for storing the music data in the device. To overcome these problems, it is demanded to compress music data.
Japanese Laid-Open Patent Publication (Kokai) No. 8-22281 discloses a method of compressing MIDI signals by analyzing MIDI signals as music data to detect a tone or a pattern which continuously occurs repeatedly, deleting a portion of the music data corresponding to the detected repeatedly occurring tone or pattern, and inserting into the music data a signal indicative of the tone or pattern being to continuously occur repeatedly in place of the deleted portion. Another method is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 9-153819, which employs a data-rearranging process in which MIDI data (composed of five data elements of tone pitch, duration, tone length, velocity, and channel number) of each tone is decomposed into the five data elements, and then pieces of data of each of the five data elements are recombined into a group of data of the data element, to thereby increase the compression ratio of a reversible (lossless) compressor in a subsequent stage.
According to the method proposed by Japanese Laid-Open Patent Publication (Kokai) No. 8-22281, however, e.g. when considering key-on events, in MIDI data, which is comprised of status information formed of information indicative of a key-on event and information indicative of a channel, key number information (7 bits), velocity information (7 bits), and gate time information (and duration information in some cases), tones identical in all these kinds of information rarely occur in succession, resulting in a low compression efficiency. Further, although a high compression ratio can be expected for compression of a type of music data containing repeated occurrences of a predetermined pattern or passage, this requires the use of a complicated algorithm for detecting long repetitive sections.
On the other hand, the technique proposed by Japanese Laid-Open Patent Publication (Kokai) No. 9-153819 is used for compressing karaoke contents in a communication karaoke system. According to this technique, karaoke contents subjected to the data-rearranging process are once downloaded into a terminal device installed in a karaoke room or at home, and then the respective groups of the five data elements are again rearranged into the original MIDI data of each tone, to be used as karaoke data. Therefore, this technique is not suitable for stream reproduction in which reproduction of music data is performed while receiving the data from a server via a network. Further, this Japanese Laid-Open Patent Publication (Kokai) No. 9-153819 only discloses rearranging elements of data, but does not propose any novel compression method.